In recent years, organic thin-film transistors using organic semiconductor materials have been intensively studied and developed.
Hitherto, organic semiconductive materials of low molecular weight have been reported, such as acene materials (e.g., pentacene) (see, for example, PTL 1 and NPL 1).
It has been reported that the organic thin-film transistors including an organic semiconductive layer formed of the aforementioned pentacene has relatively high charge mobility. However, these acene materials have extremely low solubility to common solvents. Therefore, these materials need to be vacuum-deposited to form a thin film as an organic semiconductive layer of an organic thin-film transistor. For this reason, these materials do not meet the demand in the art, which is to provide an organic semiconductive material that can be formed into a thin film by a simple wet process such as coating or printing.
As one of the acene-based materials such as pentacene, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene having the following Structural Formula (1) (see PTL 2 and NPL 2), which is a derivative of benzothieno[3,2-b]benzothiophene, is deposited on a substrate having been treated with octadecyltrichlorosilane, so that the deposited product exhibits a mobility comparable to that of pentacene (approximately 2.0 cm2/V·s) and has prolonged stability in the atmosphere.
However, this compound also needs to be vacuum-deposited similar to pentacene. Thus, this material also does not meet the demand in the art, which is to provide an organic semiconductive material that can be formed into a thin film by a simple process such as coating or printing.
The organic semiconductor materials can be easily formed into a thin film by a simple process such as a wet process, for example, printing, spin coating, ink jetting, or the like. The thin-film transistors using organic semiconductor materials also have an advantage over those using inorganic semiconductor materials in that the temperature of the production process can be lowered.
Thus, a film can be formed on a plastic substrate having a generally low heat resistance, so that electronic devices such as displays can be reduced in weight and cost. Further, the electronic devices are expected to be widely used by taking advantage of flexibility of the plastic substrate.
Moreover, 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene represented by the following General Formula (2), having liquid crystallinity and high solubility, can be applied by spin coating or casting (see PTL 2 and NPL 3). This compound is also a derivative which exhibits a mobility comparable to that of pentacene (approximately 2.0 cm2/V·s) when thermally treated at a temperature equal to or lower than the temperature at which the compound shows a liquid crystal phase (about 100° C.).
However, the temperature at which this compound shows a liquid crystal phase is relatively low; i.e., about 100° C., and the film formed therefrom may be changed through thermal treatment after film formation. Thus, this compound poses a problem in process adaptability in production of organic semiconductor devices.

In recent years, a method of producing a field-effect transistor is reported, wherein a low-molecular-weight compound having high solvent solubility is used as a semiconductor precursor, which is dissolved in a solvent and the like, and applied so as to form a film by a coating process, and then the film is transformed to an organic semiconductor film. Intensive studies have been made on methods of converting the precursor to pentacene, a porphyrin-based compound, and a phthalocyanine-based compound through retro-Diels-Alder reaction (see, for example, PTLs 3 to 5 and NPLs 4 to 7).
As described in NPL 4, the charge mobility of organic semiconductor materials depends on the orderly molecular arrangement (e.g., crystallization) in organic material films. When a vapor-deposition method is employed, the molecular arrangement of the materials in the films can be surely obtained. Meanwhile, the organic materials with molecular arrangement generally have a low solubility to an organic solvent. That is, in the organic material films, the semiconductive property and film formability (through coating) are generally in a trade-off relation.
Thus, in only one possible method for attaining both satisfactorily, after a coating film has been formed from a coating liquid containing a semiconductor precursor having a solubility-imparting group, the precursor in the coating film is converted to an organic semiconductor material. When the disclosures of these literatures are deductively considered having such meanings, it can be said that these literatures have contributed in some degree to the improvement of the technology.
However, in the above-described example, a tetrachlorobenzene molecule or other molecules are eliminated from the pentacene precursor. Here, tetrachlorobenzene has a high boiling point and is hard to be removed from the reaction system. Additionally, there is concern for its toxicity.
Also, both porphyrin and phthalocyanine require complicate syntheses, and thus are used in narrow applications. Therefore, there is a need to develop a substituent-containing compound that can be synthesized in a simple manner.
Also, it has been proposed that, by applying an external stimulus to a precursor having a high solvent-solubility and sulfonate-based substituents so that the substituents are eliminated and substituted with hydrogen atoms, the precursor is converted to phthalocyanine (see, for example, PTL 6).
However, in this method, the sulfonate-based substituents have a high polarity and thus do not have sufficient solubility to an organic solvent having no polarity. In addition, sulfonic acid (eliminated component) has a high boiling point and difficult to remove from the reaction system. Furthermore, the temperature for conversion of the precursor is relatively high; i.e., at least 250° C. to 300° C. or higher, which is disadvantageous.
Also, it has been proposed that, by applying a thermal stimulus to a nitroester so that their substituents are eliminated, the nitroester is converted to a naphthalene derivative (see, for example, NPL 8). However, nitroesters such as nitroglycerine and nitrocellulose are unstable and explosive, and thus, such compounds are difficult to store for a long period of time.
Also, it has been proposed that an alkyl group-containing carboxylate is introduced to the end β-position of an oligothiophene for imparting solubilization, and then heat is applied to eliminate the carboxylate, thereby obtaining an olefin-substituted oligthiophene or an olefin-substituted [1]benzothieno[3,2-b][1]benzothiphene (see, for example, PTLs 7 and 8 and NPL 7).
In this method, the elimination occurs by heating to about 150° C. to about 250° C., and the converted compound has at its ends olefin groups (e.g., a vinyl group and a propenyl group) which involve cis-trans isomerization due to heat or light. Thus, the resultant material is problematically degraded in purity and/or crystallinity. In addition, such highly reactive olefin end groups allow the compound to decrease in stability to oxygen or water. Furthermore, one olefin group is thermally polymerized with another olefin group at higher temperatures.
In the above-described conventional compounds, there have been problems regarding solubility of the precursors, safety of the eliminated components, the conversion temperatures, and stability of the converted compounds. Also, it is difficult to obtain desired intermediate compounds in the synthesis processes of the conventional compounds.
π-Electron conjugated compounds, having a moiety in which double bonds and single bonds are alternatingly located, have a highly extended π-electron conjugation system, and thus, are excellent in hole transportability and electron transportability. Thus, such π-electron conjugated compounds have been used as electroluminescence materials and organic semiconductor materials (see, for example, PTLs 1 and 2 and NPLs 1 and 2) as well as organic dyes and pigments. The π-electron conjugated compounds widely used involve the following problem, for example. Specifically, most of the π-electron conjugated compounds are rigid and highly planar, and thus the intermolecular interaction is very strong. As a result, these compounds have poor solubility to water or organic solvents. For example, the organic pigments made of such conjugated compounds are unstable in dispersion due to aggregation of the pigments. Also, taking an example electroluminescence materials and organic semiconductor materials made of such conjugated compounds, a wet process (using a solution) is difficult to employ since the conjugated compounds are sparingly soluble. As a result, vapor phase-film formation (e.g., vacuum vapor deposition) is required to elevate the production cost and complicate the production process which is disadvantageous. Considering the coating on a larger area and the attainment of higher efficiency, the π-electron conjugated compounds are required to be applicable to wet processes using a coating liquid previously prepared by dissolving materials in a solvent (e.g., spin coating, blade coating, gravure printing, inkjet coating and dip coating). Meanwhile, the fact that intermolecular contact, rearrangement, aggregation and crystallization are easily attained since intermolecular interaction is very strong contributes to conductivity of the compounds. In general, the film-formability and the conductivity of the obtained film are often in a trade-off relation. This is one cause making difficult to employ the π-electron conjugated compounds.
In order to overcome the above-described problems, it has been proposed that an external stimulus is applied to an organic compound precursor (including π-electron conjugated compound precursors) having reactive substituents (which impart the solubility to the precursor) to thereby eliminate the substituents to obtain a compound of interest (see, for example, PTLs 9 and 10 and NPL 9). In this method, for example, a pigment precursor having a structure in which an amino group or an alcoholic or phenolic hydroxyl group is modified with a t-butoxycarbonyl group (a t-Boc group) is heated or treated otherwise to thereby eliminate the t-Boc group. However, some limitation is imposed on the compound employable in this method, since the substituent must be bonded to the nitrogen atom or oxygen atom. In addition, further improvement has been required in terms of stability of the precursor.
Meanwhile, in recent years, intensive studies have been made on a method of applying an external stimulus to a precursor having solvent-soluble bulky substituents so that the solvent-soluble bulky substituents are eliminated, and converting the precursor to a pentacene, a porphyrin-based compound, and a phthalocyanine-based compound (see, for example, PTLs 3 and 4, NPLs 4, 5, 6 and 7).
However, in the above-described example, a tetrachlorobenzene molecule or other molecules are eliminated from the pentacene precursor. Here, tetrachlorobenzene has a high boiling point and is hard to be removed from the reaction system. Additionally, there is concern for its toxicity. Also, both porphyrin and phthalocyanine require complicate syntheses, and thus are used in narrow applications. Therefore, there is a need to develop a substituent-containing compound that can be synthesized in a simple manner.
Also, it has been proposed that, by applying external stimulus to a precursor having a high solvent-solubility and sulfonate-based substituents so that the substituents are eliminated and substituted with hydrogen atoms, whereby the precursor is converted to phthalocyanine (see, for example, PTL 6).
However, in this method, the sulfonate-based substituents have a high polarity and thus do not have sufficient solubility to an organic solvent having no polarity. In addition, the temperature for conversion of the precursor is relatively high; i.e., at least 250° C. to 300° C. or higher, which is disadvantageous.
Also, it has been proposed that an alkyl group-containing carboxylate is introduced to the end of an oligothiophene for imparting solubilization, and then heat is applied to eliminate the carboxylate, thereby obtaining an olefin-substituted oligthiophene (see, for example, PTL 7 and NPL 8).
In this method, the elimination occurs by heating to about 150° C. to about 250° C., and the converted compound has at its ends olefin groups (e.g., a vinyl group and a propenyl group) which involve cis-trans isomerization due to heat or light. Thus, the resultant material is problematically degraded in purity and/or crystallinity. In addition, such highly reactive olefin end groups allow the compound to decrease in stability to oxygen or water. Furthermore, one olefin group is thermally polymerized with another olefin group at higher temperatures.